.NET Multi-Targeting Packs for Visual Studio 2010

Microsoft provides a set of
“Multi-Targeting Packs”, to provide Microsoft Visual Studio and other
developer tools with a structured system for targeting .NET platforms.
The primary content of these targeting packs is reference assemblies,
which describe the APIs supported by a given .NET platform version.
Compilers and other tools and services can consume these assemblies to
provide a high-fidelity representation of that .NET platform version.